Password Rule Checker

Build a small password validator. Each rule should live in its own function, and the final validator should report which rules failed.

1. `hasMinimumLength(password)` should return `true` when the password has at least `8` characters.
2. `hasNumber(password)` should return `true` when the password contains at least one number.
3. `hasUppercaseLetter(password)` should return `true` when the password contains at least one uppercase letter.
4. `getFailedRules(password)` should return an array of missing rule names.
5. `validatePassword(password)` should return an object with `valid` and `failedRules`.